As it is known, an instrument cluster for vehicles has different graphic areas, which are shaped so as to provide the driver with different items of information. For example, these graphic area can display numbers, letters, marks of graduates scales, logos, etc.; moreover, some graphic areas may have a mere aesthetic function in the instrument cluster.
The graphic areas are usually defined by transparent or semi-transparent portions of a polycarbonate plate. These portions are back-lit, so as to make the corresponding items of information visible, in particular in the night light. The polycarbonate plate is normally lit by a backlighting device comprising at least one light source, namely a LED light source, and a light-guiding element, which is arranged under the polycarbonate plate and transmits light from the light source to the different zones located in the graphic areas. In this way, the light reaches the graphic areas through reflection and diffusion inside the material of the light-guiding element, so that the driver visually perceives the graphic areas in a clear fashion and without any bother. Furthermore, in many solutions, the light-guiding element is treated or processed so as to make the lighting as uniform as possible among the graphic areas arranged in different positions in the instrument cluster.
Recently, for aesthetic reasons, the need has been felt to provide graphic areas defined by openings of the polycarbonate plate, which are engaged by projecting portions of the light-guiding element.
With this type of solution, higher manufacturing costs must be accepted. As a matter of fact, for the same type of vehicle, constructors generally build instrument clusters having the same shape and the same overall dimensions, but provided with graphic areas arranged in different positions and/or with different shapes. By way of example, let's think of instrument clusters that are different because of the fact that they have revolution-counters or speed indicator instruments provided with different full scales, where the markings and the indicating numbers are hence arranged in different positions. As a result, when using a light-guiding element provided with projecting portions that define the aforesaid markings, for each type of instrument cluster it is necessary to provide a dedicated light-guiding element—and not only a dedicated polycarbonate plate.
In the solution shown in FIG. 4 of JP2006214953A, which corresponds to the preamble of claim 1, the graphic areas of the instrument cluster are defined by projecting portions of a front light-guiding element, which is arranged above a rear light-guiding element, which receives light from a light source and then transmits it to the front light-guiding element.
A face of the rear light-guiding element is provided with a plurality of recesses, which are arranged in respective positions that are aligned with the projecting portions of the front light-guiding element. These recesses allow the light flow to be uniform among the different markings or graphic areas provided in the instrument cluster. In particular, the aforesaid recesses have a substantially triangular profile, fulfil the function of a prism, namely a reflection surface, and have different depth, so as to vary the lighting among the different markings.
This solution is scarcely satisfactory, as not only the front light-guiding element, but also the rear light-guiding element must be manufactured and processed in a dedicated manner in order to obtain the different instrument clusters, since the positions of the recesses described above vary depending on the position of the markings, with a consequent increase of manufacturing times and costs.
Furthermore, the shape of the recesses is not ideal to direct the light at the markings defined by the front light-guiding element, since the result depends on the relative position between the recesses and the LED sources.